Abstracts Gears2011

The Design Challenges of Transmissions for Greener Energy Applications

Paul Bradley - Allen Gears

 

Contemporary Face Gears - A Real Alternative to Bevels?

Michael Sykes et al - Centa Transmissions

Face gear design allows a wide variety of geometries and the facility to take multiple drives off a single face. Axial freedom of the pinion, free choice of axis angles and easy assembly are additional advantages which led this type of angular gears win market shares compared to bevel gears. The presentation summarises the development of the face gear technology over the last 20 years and highlights recent improvements on geometry, stiffness and surface optimisation of Cylkro® face gears of ASSAG, Switzerland. Actual applications, such as differentials in middle class cars and power-driven vehicles as well as  multiple outputs in the field of renewable energy plants are illustrated and compared to bevel gear solutions.

International Calculation Method for Micropitting

Dr Stefan Beermann - KISSsoft AG

Micropitting is a phenomenon that, compared to pitting or root fracture, occurs relatively seldom and is often tolerable. However, due to an increasing number of high precision gears running at the technical limit, especially in the wind turbine business, the effect of Micropitting is becoming more important. It can happen in Hertzian contacts with rolling and sliding, such as on gear flanks, operating in boundary lubrication regimes. The probability of Micropitting depends mainly on the surface roughness and the properties of the lubricant.
The international calculation method for Micropitting, ISO TR 15144:2010, was recently published. It is the first official international calculation method to check for the risk of Micropitting. The German FVA published methods in the 1990s, with a first method that was only checking the situation at one contact point, and a second approach, some years later, extending the investigation over the whole flank. AGMA published a method at the beginning of this millennium for the calculation of the specific oil film thickness containing some comments about Micropitting, partly referring back to the FVA papers. The ISO TR is based on this precedent work. New in the technical report is the calculation of Micropitting safety factors. 
An ISO TR is a Technical Report, which, normally after 3 years, will become an International Standard. Over recent years, Micropitting has become a very important topic in gearbox design, especially for wind turbines. Therefore, even before the official publication, the evaluation of Micropitting risk, based on ISO 15144, was requested by some authorities, such as the Germanische Lloyd (Lloyds Register of Germany).
In this paper the outline of the ISO TR 15144 and its preceding methods is presented. The technical report presents two calculation rules, methods A and B.  Method A is based on knowledge about the values for the Hertzian pressure at  every contact point, based on an accurate calculation of the meshing of the gear pair. This should  take into consideration tooth and shaft deflections to obtain the load distribution over the flank line in every meshing position. Such a calculation is very time consuming when using FEA. Alternatively, specific analytical programs such as LDP, RIKOR or KISSsoft may be used. In either case, the use of Method A without such an advanced tool is impossible.
Method B is much simpler; the load distribution is defined for different cases such as spur or helical gears and with or without profile modifications.  However, a restriction, which arose in the last meeting of the ISO working group responsible for this topic, considerably limits the application of method B: If gears with profile modification have to be verified, the tip relief (Ca) has to correspond exactly to a proposed value Ceff. If not, the method for gears without any profile modifications has to be used. As modern gear design implies profile modifications of different kinds, this is a critical limitation for the application of method B in ISO TR 15144.   It therefore prevents the design of the profile modifications to reduce the risk of Micropitting.
The risk of Micropitting is highly influenced by profile and flank line modifications. An extension of the contact analysis in KISSsoft can systematically vary the applied modifications of a given gear set in combination with diverse torque levels.  This allows the evaluation of Micropitting risk using method A.  The results are presented in a table, showing the safety factor against Micropitting for different subsets of profile/flank modifications, depending on the torque level. Additionally, Peak-to-Peak-Transmission-Error, maximum Hertzian stress, wear etc. is also documented. This is a powerful tool to show the possibilities to reduce the Micropitting risk with profile modifications and is very helpful for the design of an optimal gear modification. An example of an application using this tool to optimize modifications will be presented.

The International Standards Process for Gears

Dr Rob Frazer - Design Unit, Newcastle University & BS Committee MCE/5 Gears

Gear Standards published by ISO are numerous, complex and appear to change without warning.  The general perception of users of these standards is they appear out of the ether, and when applied to their specific application, results don’t agree with experience.  This presentation will try and banish this perception by describing the process of how gear standards are prepared and revised within the International Standards Organisation (ISO) and how the UK national body, BSI, supports this activity.  It will show how and why you can get involved.
The BGA plays an important role in the international gear community by actively participating in the development of ISO gear standards.  It is probably one of the most important tasks it undertakes, but few people are aware of this.  The work is undertaken by volunteers nominated by the BGA and supported by small UK based working groups.  Participation in these working groups is not time consuming and it gives you the opportunity to ensure an International Standard will meet your Company requirements.
The presentation will describe the process of ISO standard development and revision.  The different types of ISO publications will be explained and some key interpretation issues will be described.
Some important limitations in the standards will be described and why it is important to continue to research and develop these areas.